This 3-year R01 proposal will examine socio-developmental and demographic risk and protective factors that characterize e-cigarette (e-cig) users, explore the concomitant use of e-cig and conventional cigarettes (cigarettes), and investigate intergenerational transmission of e-cig use and norms. Analyses will 1) identify which risk and protective factors are most predictive of e-cig use among adolescents, young adults, and adults, explore how these factors differ from tobacco risk factors and by gender and ethnicity; 2) examine the interplay of tobacco products and e-cigs, examine whether cigarette use leads to e-cig and vice versa, whether they hinder cessation attempts among smokers, and whether using e-cigs is associated with more positive norms toward tobacco; and 3) explore the degree to which parent e-cig use and norms are associated with child use and positive norms for e-cigs and cigarettes, and pose a risk for child use of other substances. The proposed project is uniquely suited to address these aims. It builds on three existing longitudinal studies: The Seattle Social Development Project (SSDP), The Intergenerational Project (TIP), and the Community Youth Development Study (CYDS). SSDP participants (n = 808) have been followed since age 10 (1985) and have been interviewed 14 times. The most recent data collection at age 39 (2014) included measures of e-cig use. TIP has followed SSDP participants who have become parents, their first-born child, and another caregiver (N = 412 families) since 2002 using an accelerated longitudinal design (child age 1 - 28 years, N = 315 have been interviewed during adolescence between ages 10-20). Parents and children have been interviewed eight times; two additional waves of data, including e-cig items, will be collected in 2017 - 2018 (funded separately). CYDS is a longitudinal panel of youth (n = 4,407; control group n = 2,002) from 24 communities across seven states which began in 2003. Participants have been interviewed 10 times from ages 10 through 23 (2016). Data at ages 21 and 23 contain e-cig modules. The proposed project is informed by the Social Development Model and accumulated knowledge about the risk and protective factors for tobacco use from the literature. The current study brings needed longitudinal investigations that span multiple developmental periods, prospectively measured tobacco use history, and the ability to look at the intergenerational transmission of e-cig use and norms. Data from these three datasets will provide insight into identifiable characteristics of e-cig users, their impact on tobacco use among adolescents (TIP children), young adults (CYDS), and adults (SSDP, TIP parents), as well as the relationship between parent and child e-cig use (TIP). By using existing longitudinal studies, the current proposal is innovative, cost- and time-effective, and is uniquely positioned to rapidly address critical gaps in our understanding of e-cigs. Findings form this study will yield viable targets for prevention and direct implications for policy and regulation.